MySql內部加入訂單

Question

我有兩個表USERS和VOTES;

我做兩個表與內部聯接以下代碼;

select *, count(*) as people_count from users inner join votes 

on users.id=votes.user_id and votes.choise_id=1 

group by users.country, votes.choise_id 

order by votes.time desc; 

我得到波紋管結果

我想最後評論說，Erric的，不是湯姆的

你可以從下面的鏈接

運行代碼http://www.sqlfiddle.com/#!2/69cc95/3

感謝您的幫助

問候

Answer 1

我找到一個sollution 但我不知道它是否足夠快

select * , count(*) as people_count from 

(select * from votes v 
inner join users u 
on v.user_id=u.id 
where v.choise_id=1 
order by v.time desc 
) t 

group by t.country 

你可以從下面的鏈接

http://www.sqlfiddle.com/#!2/69cc95/40

Answer 2

如果你想一排用最新的值，那麼你應該使用not exists而不是group by：

select *, 
     (select count(*) from votes where v.choise_id = 1) as people_count 
from users u inner join 
    votes v 
    on u.id = v.user_id and v.choise_id = 1 
where not exists (select 1 
        from users u2 join 
         votes v2 
         u2.id = v2.user_id 
        where u2.country = u.country and 
         v2.choise_id = v.choise_id and 
         v2.time > v.time 
       );